The present invention relates to a method for the manufacture of an article having a first and a second surface region of different coefficients of friction using a treatment chamber suitable for the carrying out of PVD processes.
The invention is concerned with a new concept belonging to the invention to the effect that it must be possible to provide a relatively high coefficient of friction on a first surface region and a relatively low coefficient of friction on a second surface region by means of a PVD process at a reasonable cost.
Examples for applications where such a process could be useful are, for example:
links of driving chains which are installed in infinitely variable drives;
bearings which should have a high coefficient of friction on the outside so that they can be held in a housing without slipping, but which should have a lower coefficient of friction on the inside so that they can provide the journalling of a shaft or another component with extremely low wear properties; and
fastener elements and washers for fastener elements. For example, a washer for a nut should have a higher coefficient of friction on the surface facing the component so that the washer does not turn on the component and damage the component when the nut is tightened, while the coefficient of friction for the pair washer/nut should be small so that the nut can be properly tightened, for which a relative rotary movement is required between the nut and the stationary washer.
In order to satisfy this object, the initially named method is characterized in accordance with the invention by at least two PVD coating processes being carried out at the same time in the treatment chamber, with the first PVD coating process being carried out with a comparatively directed vapor flux from one target or from a plurality of targets which can comprise one or more of the elements B, Si, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W, carbides of the said elements and carbon and with the second PVD coating process being carried out with a comparatively less directed or non-directed vapor flux of carbon.
The invention is based on the recognition that carbides consisting of the said elements and carbon, in particular coatings with incorporated hydrogen, have a coefficient of friction which is dependent on the composition of the layer. For example, a coating B4Cxe2x80x94C:H with 55 atomic % of boron and consequently 45 atomic % of C:H has a coefficient of friction of 0.2, whereas the same coating with 80 atomic % of boron has a coefficient of friction of 0.9.
A composition-dependent coefficient of friction can be found in all these elements, although the exact compositions required to produce clearly different coefficients of friction differ in each case. For tungsten, for example, a composition of Wxe2x80x94C:H with 50 at % W has a high coefficient of friction, whereas with 15 at % W and a consequently higher proportion of carbon a low coefficient of friction results.
As a result of the deposition of the carbide layer from a comparatively directed vapor flux, this coating is preferably effected on the surface region which is perpendicular to and facing the vapor flux, whereas a considerably thinner coating is formed on surfaces which are not perpendicular to the vapor flux, i.e. are not facing it.
By the carbon vapor flux being comparatively less directed or non-directed, i.e. de facto forming a carbon cloud, the carbon is present at all surface regions equally with an approximately constant distribution density. As the carbon on the first surface region mixes with a larger supply of carbide molecules, a coating with relatively little carbon is effected there, whereas a high carbon concentration is achieved on the second surface region where comparatively few molecules of the carbide are deposited due to the directed properties of the vapor flux.
Thus, different carbon contents are present on the two said surface regions which result in the different coefficients of friction on these surface regions desired in accordance with the invention.
While, strictly speaking, it is not necessary to provide any movements of the articles to carry out the method in accordance with the invention, particularly when only a single article is to be coated, it is, however, advisable for reasons of process economy and for the generation of a uniform coating to arrange the object or a plurality of objects on a rotary support in a manner known per se.
The two PVD coating processes can be realized in a variety of ways.
It is possible, for example, to carry out the two PVD coating processes by using a single PVD coating apparatus. To realize this, one proceeds such that a metal vapor or metal and carbide ions are generated with one or more targets using a cathode sputtering process or an arc discharge vaporization process and directed onto the article as a directed vapor flux, with the cathode sputtering process or the arc discharge vaporizing process resulting in or contributing to the generation of a plasma in the treatment chamber and thereby to the generation, from a hydrocarbon atmosphere present in the treatment chamber, of carbon ions and molecules in the form of a less directed or non-directed vapor flux, which is at least partially incorporated into the coating being formed on the article.
The carbon ions and molecules are thus generated in a plasma which is anyway present in the cathode sputtering mode or the arc discharge vaporization mode due to the operation of a cathode (target). This plasma can also be expanded by the use of selected magnetic fields in order to increase the supply of carbon ions and molecules and above all to bring it into the region of the article. The magnetic fields can also be the magnetic fields which are needed for the operation of the cathode as an imbalanced magnetron.
Another possibility of carrying out the two PVD coating processes is to use first and second PVD apparatuses in the treatment chamber, with the arrangement being able to be chosen so that at least a first PVD coating apparatus and a second PVD coating apparatus are used in the treatment chamber, with the first PVD coating apparatus generating a comparatively directed vapor flux from a target or a plurality of targets consisting of one or more of the elements B, Si, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W, carbides of the said elements and carbon, and the second element generating a comparatively less directed or non-directed vapor flux of carbon.
To carry out the method in accordance with the invention in accordance with the first variant, the article or plurality of articles can be arranged on a rotary support and be moved through the comparatively directed vapor flux of the first PVD coating apparatus by a rotary movement of the support, with said rotary movement being either a simple rotary movement so that the article or each article has a given, at least substantially constant, effective orientation with respect to the said vapor flux, or a complex rotary movement, for example a rotary movement in which the article or each article is rotated around its own axis, with the individual rotary movements being synchronized in the case of a more complex rotary movement such that the article or each article also has a given, at least substantially constant, effective orientation with respect to the said vapor flux.
By the effective orientation of the article always being identical with respect to the directed vapor flow of the first PVD coating apparatus, the first surface region is preferably coated. By the vapor flux of carbon being less directed, it is present at all surface regions, whereby the different compositions of the first and second surface regions are created, as described above. The first PVD coating apparatus generating a comparatively directed vapor flux can be one or more of the following coating apparatuses:
an arc discharge vaporization device;
a cathode sputtering device, for example in the form of a magnetron or an imbalanced magnetron;
a linear ion source, in particular a source of this kind for the generation of carbon ions such as a plasma generating source with a heating filament.
The second PVD coating apparatus generating a comparatively less directed or non-directed vapor flux is one or more of the following coating apparatuses:
an ion source, in particular for the generation of carbon ions from a gaseous hydrocarbon supplied to the treatment chamber;
a plasma source, for example with a plasma generated from a microwave source, in particular a high-power microwave source.
A second way of satisfying the object underlying the application using a method of the kind initially mentioned is characterized in that at least a first PVD coating apparatus and a second PVC coating apparatus are used in the treatment chamber, in that the article or plurality of articles is arranged on the rotary support, in that the or each article is furthermore rotatable around its own axis on the rotary support and in that the rotary movement around the or each article""s own axis and the rotary movement around the axis of rotation of the support are synchronized such that the first surface region is coated preferably by the first coating apparatus and the second surface region preferably by the second coating apparatus.
In this variant of the invention, one works mainly with two directed vapor fluxes: on the one hand of metal, B and/or Si atoms or ions and carbon which result in the formation of a carbide on the article, and on the other hand of carbon, with it being possible here to generate different carbon contents, and thus also the desired different coefficients of friction, on the first and second surface regions thanks to the special synchronization of the rotary movements of the article(s).
With a carefully selected procedure, it is possible to ensure that a multi-layer coating is formed on the second surface region; said multi-layer coating being described exactly in the simultaneously filed German patent application 10005614.8 with the title xe2x80x9cA method for the manufacture of coatings and of an articlexe2x80x9d, in which an extremely hard, wear-resistant and elastic surface with a low coefficient of friction can be generated.
The contents of this simultaneously filed application are also made part of the disclosure of the present application by way of the above reference.
The invention is described in more detail in the following with reference to embodiments with and to the drawings.